The present invention relates to focused particle beam systems and methods for processing a workpiece, e.g., etching and imaging a cross-section of a workpiece.
Present focused ion beam (FIB) systems typically include an ion beam column oriented normal to the workpiece and a tilting work stage. Such systems can include an electron column offset from the normal to the workpiece. To image a cross-section of a workpiece using an ion column, existing systems etch a cavity in the workpiece and tilt the stage so that the ion beam can impinge on a side wall of the cavity.
Existing FIB systems which incorporate a tilting stage experience several problems. A tilting work stage, which is large relative to many of the other components of a FIB system, causes the system to be relatively bulky. Such a large bulk is disadvantageous because cleanroom fabrication space is expensive. A tilting work stage also causes an FIB system to be unstable because a tilting work stage can make an FIB system susceptible to low frequency vibration and gravity sag, as discussed further below. Disadvantageously, the vibration of and the changing configuration of a tilting work stage can interfere with the performance of a system component, such as a laser interferometer. Laser interferometry can be used to assist in accurate monitoring of the position of a workpiece.
Low frequency vibration can occur when a massive object, such as a tilting stage, is supported by bearings and held steady with a mechanism that behaves like a spring. Low frequency vibration reduces resolution of a focused particle beam system by adding uncertainty in the determination of the location of the target point, i.e., where the ion beam interacts with the workpiece.
When a large work stage assembly is tilted, gravity can bend components of the work stage assembly and the workpiece. Such bending is termed gravity sag. It is difficult to monitor gravity sag. Thus, gravity sag can lead to inaccuracy in determining the positions of the work stage and of the workpiece. Such inaccuracy can reduce the resolution of a focused particle beam system.
Existing configurations of FIB systems restrict access to the workpiece by other elements, such as an optical microscope. Further. existing systems do not allow for optimization of the working distance of particular ion and electron columns. In existing configurations with a focused ion beam oriented normal to the workpiece and an electron beam offset with respect to the normal, one can not achieve working distances that optimize the characteristics, e.g., resolution and current density, of the ion and electron beams, because the work stage and the tip of the ion column and the tip of the electron column physically interfere with each other.
Accordingly, it is an object of the present invention to provide improved focused particle beam systems and methods for processing, e.g., etching and imaging a cross-section of a workpiece.
It is another object of the invention to reduce the footprint of a focused particle beam system.
It is another object of the invention to improve the stability of the work stage assembly of a focused particle beam system.
It is another object of the invention to improve the accuracy of a focused particle beam system.
It is another object of the invention to provide a focused particle beam system that allows for concurrent optimization of the working distances of a particle beam column and an electron beam column, the columns being oriented so that their target points are substantially coincident.
It is another object of the invention to provide a focused particle beam system that allows greater access to the workpiece by additional system elements such as an optical microscope.
Other objects of the invention will in part be obvious and in part will appear hereinafter.
One version of a particle beam system for interacting with a workpiece according to this invention, has a housing and an element for processing a workpiece contained in the housing. The processing element includes a work stage assembly and a first particle beam source. The work stage assembly is adapted a) for supporting the workpiece, b) for translating the workpiece along a first axis, c) for translating the workpiece along a second axis perpendicular to the first axis, and d) for rotating the workpiece about a third axis perpendicular to both the first axis and the second axis. The work stage assembly has a work stage axis substantially parallel to the third axis.
The first particle beam source interacts with the workpiece supported by the work stage assembly. The first particle beam source is located above the work stage assembly and has a first particle beam axis. The first particle beam source is oriented so that the first particle beam axis forms an acute angle with the third axis. Thus, the particle beam system can etch and image a vertical cross-section of the workpiece without offsetting the work stage axis from the third axis.
Workpieces or samples, such as wafers containing semiconductor devices, can contain features or structures having aspect ratios of 15:1. Thus, when crosssectioning and imaging the cross-section of a workpiece containing such features or structures, the cross-section should be sufficiently vertical such that an individual feature""s aspect ratio is accurately reflected in the cross-section.
Further, for the purposes of this application, one axis is defined as offset relative to another axis when the one axis forms an acute angle with respect to the other axis.
For illustration purposes only, and not to be taken in a limiting sense, the above-mentioned first and second axes can define a horizontal plane and the above-mentioned third axis can be a vertical axis. In this case, the work stage assembly can be adapted a) for supporting the workpiece in a horizontal plane, b) for translating the workpiece along a forward/backward direction, c) for translating the workpiece along a side to side or along a right/left direction, and d) for rotating the workpiece about the vertical axis. The work stage assembly has a work stage axis substantially parallel to the vertical axis. The first particle beam source has a first particle beam source axis oriented to form an acute angle with the vertical axis. Thus the particle beam system can etch and image a vertical cross-section of the workpiece without offsetting the work stage axis from the vertical axis.
There are several embodiments of this version of a focused particle beam system according to the invention. The first particle beam axis can form an angle of about forty-five degrees with the third axis. The system can further include a second particle beam source for interacting with the workpiece, located above the work stage assembly. The second particle beam source can have a second particle beam axis. In one embodiment, the second particle beam source is oriented so that the second particle beam axis is substantially parallel to the third axis. In another embodiment the second particle beam source is oriented so that the second particle beam axis is offset relative to the third axis.
In another embodiment, the system can further include an electron beam source for interacting with the workpiece. The electron beam source is located above the work stage assembly and has an electron beam axis. The electron beam source is oriented so that the electron beam axis is selectively offset relative to the third axis.
There are still other embodiments of this version of a focused particle beam system according to the invention. The system can be configured so that the first particle beam axis and the electron beam axis each form an angle of about forty-five degrees with the third axis. Further, the system can be configured so that the first particle beam axis and the third axis form a first plane and the electron beam axis and the third axis form a second plane substantially perpendicular to the first plane. This system configuration is advantageous because the system can etch a vertical cross-section of a workpiece using the first focused particle beam source and can image the vertical cross-section using the electron beam without rotating the workpiece.
The system can be configured so that the work stage assembly includes a laser interferometer element for assisting in the accurate determination of the position of the workpiece. The laser interferometer can include a laser source, a beam splitter, at least one reference mirror, and at least one test mirror. The laser source directs laser radiation along a path in a first direction. The beam splitter is located in the path of the laser radiation from the laser source and transmits a first part of the laser radiation along the first direction, and reflects a second part of the laser radiation along a second direction. The reference mirror reflects back to the beam splitter the first transmitted part of the laser radiation. The test mirror reflects back to the beam splitter the second reflected part of the laser radiation and is located on said work stage assembly. Thus, the beam splitter combines the first transmitted part and the second reflected part of the laser radiation to form interference fringes that assist in the determination of the position of the workpiece.
The system can also be configured to include a gas injection source or an optical microscope or both. The gas injection source typically has a gas injection nozzle located above and in selected proximity to the workpiece. The optical microscope has an optical microscope axis and is oriented so that the optical microscope axis is substantially parallel to the third axis. One can use the optical microscope for so-called top-down wafer navigation.
The system can also be configured to include a work stage assembly that rotates more than twenty-five degrees, more preferably at least forty-five degrees and most preferably at least ninety degrees.
According to another version of the invention, the first particle beam source for interacting with the workpiece is tiltable from a first position, where the first particle beam axis is substantially parallel to the third axis, to a second position, where the first particle beam axis forms an angle with the third axis. With this arrangement, the particle beam system can etch and image a vertical cross-section of the workpiece without offsetting the work stage axis from the third axis.
A method for using a particle beam system to interact with a workpiece, according to one version of the invention, includes the steps of a) providing a particle beam system, b) placing the workpiece on a work stage assembly, and c) etching with the focused particle beam source a first cavity in the workpiece to expose at least a portion of at least one structure contained in a vertical crosssection of the workpiece.
The step of providing the particle beam system can include providing a work stage assembly adapted a) for supporting a workpiece, b) for translating the workpiece along a first axis, c) for translating the workpiece along a second axis perpendicular to the first axis, and d) for rotating the workpiece about a third axis perpendicular to both the first axis and the second axis. The work stage assembly has a work stage axis substantially parallel to the third axis.
The step of providing the particle beam system can also include providing a first particle beam source for interacting with the workpiece. The first particle beam source is located above the work stage assembly. The first particle beam source has a first particle beam axis. The first particle beam source is oriented so that the first particle beam axis forms an acute angle with the third axis.
Thus, the particle beam system can etch and image a vertical cross-section of the workpiece without offsetting the work stage axis from the third axis.
The step of providing a particle beam system can further include the step of providing an electron beam source for interacting with the workpiece. In this embodiment the electron beam source is located above the work stage assembly and has an electron beam axis. The electron beam source is oriented so that the electron beam axis is selectively offset relative to the third axis.
The step of providing a particle beam system can further include the step of providing the electron beam source and the first particle beam source with the first particle beam axis and the electron beam axis each forming an angle of about forty-five degrees with the third axis. Further, the first particle beam axis and the third axis can form a first plane and the electron beam axis and the third axis can form a second plane such that the first plane is substantially perpendicular to the second plane.
The above method can further include the step of imaging the vertical cross-section of the workpiece using the electron beam source.
The above method can further include the step of etching a second cavity in selected proximity to the first cavity so as to produce a transmission electron microscope (TEM) sample wall or lamella separating the two cavities. The TEM lamella can have first and second opposed sides. The first side faces the first cavity and the second side faces the second cavity. The method can further include the steps of bombarding the second side of the TEM lamella with electrons from the electron gun, and monitoring the change in secondary particle emission from the lamella while etching the second cavity to monitor the thickness of the lamella.
The method described above can further include the step of rotating the workpiece ninety degrees about the third axis to expose the vertical cross section to the first particle beam source subsequent to the etching step. Subsequent to the rotating step, the focused particle beam system can image the vertical crosssection of the workpiece using the focused particle beam source.
Another version of a particle beam system for interacting with a workpiece according to the invention includes a work stage assembly for supporting a workpiece and for orienting the workpiece in a plane. The work stage assembly has a support element adapted for translating the workpiece along a first axis, and for translating the workpiece along a second axis perpendicular to the first axis. The support element has a first side and a second side, and a positioning assembly coupled to the first side of the support element and adapted for rotating the support element and the workpiece about a third axis perpendicular to both the first axis and the second axis, such that the workpiece can be seated on the second side of the support element and translated in a plane and rotated about the third axis normal to that plane.
The system further includes a first particle beam source for interacting with the workpiece. The workpiece is supported by the work stage assembly. The first particle beam source is located above the work stage assembly and has a first particle beam axis. The first particle beam source is tiltable from a first position with the first particle beam axis substantially parallel to the third axis, to a second position with the first particle beam axis forming an acute angle with the third axis. Thus, the particle beam system can etch and image a vertical cross-section of the workpiece without tilting the work stage axis relative to the third axis.
These and other features of the invention are more fully set forth with reference to the following detailed description and the accompanying drawings.